Non-invasive, low pin count test circuits and methods utilizing emulated stress conditions

ABSTRACT

A method of testing an internal block of an integrated circuit includes testing an internal block under a selected operating condition by setting a selected operating parameter to a value emulating operation of the internal block under another operating condition to detect potential failure of the internal block under the another operating condition.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application for patent is a continuation in part of the following application for patent:

[0002] Pending U.S. patent application Ser. No. 10/027,187 (Attorney Docket No. 1248-CIC [2836-P176US]) filed Dec. 20, 2001 by inventors Kejariwal, Ammisetti, Thomsen and Melanson and entitled “NON-INVASIVE, LOW PIN COUNT TEST CIRCUITS AND METHODS”, currently pending.

FIELD OF INVENTION

[0003] The present invention relates in general to integrated circuits and in particular, to non-invasive, low pin-count test circuits and methods utilizing emulated stress conditions.

BACKGROUND OF INVENTION

[0004] Testing integrated circuits typically involves the evaluation of important parameters at various operational levels under differing operating conditions, such as temperature. For example, the overall operation of the chip may be tested in addition to the specific testing of particularly critical circuits or circuit blocks. This testing is especially important with respects to complex integrated circuits in which the overall operation of the device may fall within specifications, but the operation of one or more internal circuit blocks is nonetheless only marginal.

[0005] The actual implementation of an efficient test protocol for a given chip is a non-trivial task subject to many variables. Among other things, if on-chip test circuitry is to be used, that test circuitry must be non-invasive. In other words, the operation of the test circuitry should not in itself alter any of the critical operating parameters of the device or disturb a critical signal path. Additionally, depending on packaging limitations, it is not always practical to provide sufficient pins for observing all the internal circuits requiring test. Further, notwithstanding the problem of access, ways of triggering the internal circuitry test mode must be provided. Finally, some decision must be made as to which parameters and nodes are to be tested.

[0006] Given the importance of testing at various operational levels of an integrated circuit, improved testing techniques are required. These techniques should be non-invasive, neither disturbing critical signal paths nor dictating undue changes in the physical configuration device or packaging. They should be flexibly amenable to the testing of various nodes and parameters on the integrated circuit in a time-efficient fashion.

SUMMARY OF INVENTION

[0007] The principles of the present invention allow potential failures of an integrated circuit under stress to be detected from testing performed under nominal (ambient) test conditions. According to one particular embodiment of these principles, a method of testing an internal block of an integrated circuit is disclosed which includes testing an internal block under a selected operating condition by setting a selected operating parameter to a value emulating operation of the internal block under another operating condition to detect potential failure of the internal block under the another operating condition.

[0008] Advantageously, the principles of the present invention allow for the testing of embedded circuits in an integrated circuit chip using a streamlined test protocol. Specifically, by varying a selected operating parameter, such as bias current, stress conditions for one or more of the embedded blocks are emulated. The integrated circuit chip is then tested in nominal test conditions to detect embedded blocks, which have a probability of failing under those stress conditions. Hence, the need to perform testing under actual stress conditions to detect marginal or failing parts is substantially reduced or eliminated.

BRIEF DESCRIPTION OF DRAWINGS

[0009] For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0010]FIG. 1 is a operational block diagram of an exemplary multipath feedforward operational amplifier embodying the present inventive principles;

[0011]FIG. 2 is a operational block diagram of one exemplary circuit for stepping the supply current to the operational amplifier of FIG. 1 in response to selected test conditions;

[0012]FIG. 3 is a timing diagram illustrating the typical operating regimes of the operational amplifier of FIG. 1;

[0013]FIG. 4A illustrates an exemplary power current profile illustrating the modulation of the power supply current during the test phase shown in FIG. 3;

[0014]FIGS. 4B-4F are voltage versus time diagrams of exemplary timing signals controlling the operating regimes of FIG. 3 as generated by the state machine of FIG. 1;

[0015]FIG. 5 is an electrical schematic diagram of an exemplary power detect circuit suitable for generating the power detect control signal of FIG. 4B;

[0016]FIG. 6 is an electrical schematic of a selected one of the test circuit blocks shown in FIG. 1;

[0017]FIG. 7 is an electrical schematic diagram of an exemplary chopper-stabilized integrator suitable for use in selected ones of the integrator stages of FIG. 1;

[0018]FIGS. 8A and 8B are current profiles illustrating alternate methods of modulating the power supply current in response to selected test conditions;

[0019]FIG. 9 is a operational block diagram of a second exemplary multipath feedforward operational amplifier embodying the present inventive principles;

[0020]FIG. 10 is an electrical schematic diagram of an exemplary variable bias current generator suitable for utilization in the representative operational amplifier of FIG. 9;

[0021]FIG. 11 is a timing diagram illustrating the typical operating regimes of the operational amplifier of FIG. 9; and

[0022]FIGS. 12A-12G are voltage versus time diagrams of exemplary timing signals controlling the operating regimes of FIG. 11 as generated by the state machine of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in FIGS. 1-12 of the drawings, in which like numbers designate like parts.

[0024]FIG. 1 is a operational block diagram of a feed-forward operational amplifier 100, fabricated on a single chip, and embodying the principles of the present invention. (Opamp 100 is only one of a number of possible applications of these principles, which are particularly useful in instances where testing of deeply embedded circuits is required and/or the number of pins or pads available for parameter observation is limited).

[0025] Opamp 100 is based on five (5) integrator stages 101 a to 101 e. In the preferred embodiment, integrator stage I1 is chopper-stabilized, as discussed in further detail below. A set of summers 102 a to 102 c implement the feed-forward operation. The primary data path also includes ¼ attenuator 103 and {fraction (1/32)} attenuator-low pass filter (LPF) 104.

[0026] According to the present inventive principles, three test blocks 105 a to 105 c are provided to monitor three selected nodes representing corresponding state variables in opamp 100. In the preferred embodiment, the data paths through opamp 100 are differential and test blocks 105 a to 105 c, which monitor the differential voltage between the conductor pairs, although other parameters can also be monitored. Electrical parameters such as voltage and current can also be monitored in embodiments using single-ended data paths. In the present example, differential voltages exceeding a predetermined level are deemed to be an error, and the supply current is modulated as a flag.

[0027] Timing and control of the chopper stabilization of integrator I1 and test block is effectuated with an on-chip oscillator 106. A state machine 107 generates the test control signals described in detail below.

[0028] A high-level operational block diagram of current modulation circuitry 200 which steps the supply current during test mode operations is shown as FIG. 2. Conceptually, current modulation circuitry 200 comprises a plurality of parallel binary-weighted current sources 201 a to 201 d and associated loads 202 a to 202 d. Current source 201 a is activated during the test mode calibration phase by the control signal Cal while current sources 201 b to 201 d are activated by the output signals from test blocks 105 a to 105 c. Current sources 201 a to 201 d are deactivated in the normal mode, i.e., at the end of the test mode phase.

[0029] In the preferred embodiment, test mode operations are observed by monitoring power supply current. For this embodiment, the test mode phase is approximately 7 millisecond in duration and is divided into the time intervals shown in the timing diagram of FIG. 3. During the first interval, of about 5 millisecond (i.e. between times t₀ and t₁, the internal circuits of op-amp 200 and the external testing instruments are allowed to stabilize to a steady state. This stabilizing time is followed by a second calibration interval of approximately 2 msec. During the first 1 millisecond of the calibration interval, (i.e. between time t₁, and t₂.), the power supply current is stepped to a calibration current level. Then, in the last 1 millisecond of the calibration interval, (i.e. between time t₂ and t₃.), the power supply current is stepped as an indication of possible error locations.

[0030] An exemplary power current profile during test mode is shown in FIG. 4A. The associated timing and control signals are shown in FIGS. 4B-4F, where FIG. 4B shows the power supply voltage profile, FIG. 4C shows the signal Power_Detect ramping-up with the power supply, and FIGS. 4D-4F illustrate exemplary state machine—generated timing signals activating test blocks 105 a to 105 c and calibration current source 201 a. Specifically, state machine output signal TESTMODE1 indicates that the test mode is active, CAL times the 2 millisecond calibration interval and TESTMODE2 times the final 1 millisecond during which detected errors are identified.

[0031] During the first 5 millisecond interval, the current is higher than the nominal value (e.g. room temperature value) since the test mode circuitry is running and therefore requiring power. This interval is followed by a step of 80 microamps of calibration current for 2 msec. As shown in FIG. 2, the power supply current stepping is preferrably done by turning-on one or more parallel current sources 201. The calibration current is selected to provide a reference against which the error current steps are measured; if the calibration current source load varies from the specified nominal value for a given device due to fabrication process variations, then the error step current source loads on the same chip should vary similarly from their nominal values such that the absolute relationships between steps remains essentially the same.

[0032] In the last 1 millisecond of the test mode (e.g. the error phase from time t2 to t3 in FIG. 3), detected errors, if any, are flagged by an additional current step above the calibration level. In the present three test node example, binary weighted currents of 0, 40, 80 and 160 microamps are used to indicate the results, although other current magnitudes could be used depending on the particular application. The error step is the sum of the individual error currents representing each of the detected errors. For example, if two errors are detected, one represented nominally by 40 microamps and the other nominally by 160 microamps, then the total current step above the calibration level will be nominally 200 microamps. From the external observation point, a measured step of 200 microamps can be uniquely decoded as 40 and 160 microamps parts, representing the designated errors. If no error is detected, then the profile is substantially flat after the 80 microamps calibration step. After the test mode is complete, the power supply current returns to its nominal state current requirement for normal mode operations.

[0033] Inducing the test mode when no extra pins are available for this purpose is another problem addressed by the inventive principles. Preferably, a state machine is used which generates the control signals CAL, TESTMODE1 and TESTMODE2. The state machine is activated by power-detect circuitry 500 shown in FIG. 5. Power-detect circuitry 500 generates a pulse when the power supply voltage exceeds a selected threshold, preferably 2vt of the transistors or, for example, approximately 1.4 volts for transistors with a threshold of 0.7 volts.

[0034] While there are numerous parameters which can be tested in accordance with the inventive principles, the three parameters being tested are: (1) the differential voltages at the three test nodes; (2) oscillator frequency; and (3) chopper operation.

[0035] In the illustrated embodiment, test blocks 105 a to 105 b preferably test the offset voltage at various points along the differential data path. One test block 105 (e.g. either 105 a, 105 b, or 105 c) is shown in further detail in FIG. 6. (In this embodiment, the same voltage detection circuit is used for each test node, although the comparator limits may differ.)

[0036] Test circuitry 105 includes a difference amplifier including a differential pair of transistors 601 a,b responding to the input signals V_(IN)− and V_(IN)+ respectively and operating from a current source 603. Corresponding transistors 602 a.b are biased such that they operate in the triode (non-saturation) region when transistors 601 a,b have no differential input voltage (i.e. V_(IN)−=V_(IN)+). The common nodes represent the outputs TESTOUTM and TESTOUTP, which have a low voltage swing of between 0.2 to 0.5 volts. The outputs of the difference amplifier TESTOUTM and TESTOUTP are input into a logic-OR gate 604 which generates TESTOUTX, where x is between 1 and 3 and represents the corresponding test block (node) 105 a,c. A voltage difference between V_(IN)− and V_(IN)+ divides the current from current source unevenly between transistors 601 a and 601 b. The magnitude of the voltage difference at the gates of transistors 601 a and 601 b determines the direction and magnitude of the current split. In particular, if the difference between V_(IN)− and V_(IN)+ is sufficiently large, then most of the current is sunk through either transistor 601 a or 601 b and the corresponding voltage of TESTOUTP or TESTOUTM increases enough to cause OR gate 604 to generate the ultimate output signal TESTOUTX indicating an excessive imbalance at the node being tested.

[0037] In the preferred embodiment, each differential voltage detector is tuned by changing the current sources 603 and/or the sizes of transistors 601 a,b. Advantageously, the differential voltage detectors shown in FIG. 6 sense node voltages without interfering with signal transmission through the tested nodes. Additionally, the preferred circuitry is turned-off (with the PDTEST signal) when the normal mode of operation begins.

[0038] The oscillator frequency is derived by a time measurement over the last 2 millisecond interval of the test mode. Specifically, the current steps at times t1 and t3 are used to define the 2 millisecond period over which counter 107 or similar circuit is used to count periods of the oscillator. The oscillator frequency can be derived from the time measurement between t1 and t3.

[0039] In the preferred embodiment of op-amp 100, integrator 101 a (I1) is based on a chopper-stabilized amplifier as shown in FIG. 7, where the common mode feedback is represented by “CMFB”. Chopper-stabilized amplifiers normally and ideally chop their internal offsets. Hence, to test integrator I1, an offset V_(offset) is introduced at one of the integrator differential inputs pair transistor. If no difference is detected between the output offset voltage either during the 7 millisecond test mode or normal operation, then the chopper is functioning correctly.

[0040] It should be recognized that there are a number of alternate ways in which supply current can be modulated, two of which are shown in FIGS. 8A and 8B. Generally, the number of current levels needed to encode all possible error combinations is 2^(n) where n is the number of state variables to be monitored. In other words, when n increases, the required levels increase exponentially, thus limiting n to 3 or 4.

[0041] In one alternate time-multiplexed encoding, n is divided into smaller numbers and then each number is encoded. For example, if n is 4, the division can be made into two sequences and then each sequence coded as described above. The typical current waveform may look as shown in FIG. 8A.

[0042] In another embodiment, pulse width modulation can be used to modulate power supply current with the code. Advantageously, this technique can be used to monitor a large number of state variables in short testmode times. If the number of state variables to be monitored is large, then it can also be divided into smaller groups and a pulse width modulated sequence can be used for each number as shown in FIG. 8B.

[0043] Additionally, there are alternate ways in which the test mode can be induced. In any event, the conditions or mechanism which induces the self test mode should not be normally present or occurring during normal operation of the op amp. Moreover, the self test should only last for a short time (e.g., in this example the duration of the self test is for 7 milliseconds). In one alternate embodiment, both the differential input pins (e.g. iNPUT signal in FIG. 1) are connected momentarily to 0.3 volts below the lowest power supply voltage to the chip. Internally, a simple comparator circuit senses the voltage and triggers the state machine as described above. (This voltage should be lower than the lowest power supply voltage but higher than −0.7 volts at which voltage protection diodes at input pins (not shown) start conducting and should not be asserted during normal operation). A variation of this technique is to keep the input voltage lower for a specified time period to reduce the possibility of accidental inducement o the test mode more.

[0044]FIG. 9 is a block diagram of a multiple-stage feed-forward operational amplifier 900 according to a further embodiment of the principles of the present invention. Operational amplifier 900 includes the structures described above with respect to operational amplifier 100, along with a bias generator 901 which provides bias currents BIASI1, BIASI2, BIASI3, BIASI4, BIASI5, and BIASLPF respectively to integrators stages I1-I5 (blocks 101 a-101 e), and the lowpass filter (LPF) of attenuator-lowpass filter block 104. Bias generator 901 provides the bias currents BIASI1, BIASI2, BIASI3, BIASI4, BIASI5, and BIASLPF during normal operations of operational amplifier 100, and during testing, allows these bias currents to be varied in order to efficiently detect marginal or failing internal circuit blocks.

[0045] Specifically, in the test mode, bias generator 901 allows the internal nodes of operational amplifier 100 to be tested under nominal test conditions (e.g. room temperature) using bias currents predicted to emulate the worst case bias conditions expected to occur while operational amplifier 100 is under stress (e.g. high or low operating temperature). Once the bias currents BIAS1, BIAS2, BIAS3, and BIASLPF have been varied, offset voltage tests, such as those described above with respect to operational amplifier 100, are performed to verify internal operations of amplifier 900. Consequently, marginal internal blocks, such as integrators I1-I3 and the lowpass filter of block 104, which may fail under stress conditions can be efficiently detected under nominal test conditions.

[0046] An exemplary embodiment of bias generator 901 is shown in detail in the electrical schematic diagram of FIG. 10. While bias currents BIASI1, BIASI2, BIASI3, and BIASLPF are shown being generated in the embodiment of FIG. 10 for integrators I1-I3 and the lowpass filter of attenuator-LPF block 104, the illustrated concepts may be extended to the generation of variable bias currents BIASI4 and, BIASI5, to integrators I4-I5, and other on-chip circuitry as well.

[0047] A band gap generator 1001 establishes a reference current through n-channel transistors 1003 a and 1003 b. Transistors 103 generates the bias voltage for cascade transistors 102 a, 106 a and 107 a. The base bias currents BIASI1, BIASI2, BIASI3, respectively for integrators I1, I2 and I3 of FIG. 9 are generated, for both normal and test mode conditions, by current mirror blocks 1004 a-1004 c. Normal test and stress test mode base bias current values for BIASLPF are generated for the lowpass filter by current mirror block 1005. The operation of current mirror blocks 1004 a-1004 c and 1005, which will be described further below, is essentially the same, although in the illustrated embodiment the test mode control logic for current mirror block 1005 is the inverse of that controlling current mirror blocks 1004 a-1004 c.

[0048] Current mirror blocks 1004 a-1004 c and 1005 each include a pair of n-channel transistors 1006 a-1006 b which mirror the reference current through transistors 1002 a-1002 b to set the corresponding bias current BIAS1, BIAS2, BIAS3, or BIASLPF during normal and test mode operations. In addition, each current mirror block 1004 a-1004 c and 1005 includes a second pair of transistors 1007 a-1007 b, which selectively mirror the reference current through transistors 1002 a-1002 b during the stress test mode. For current mirror blocks 1004 a-1004 c, the current paths through transistors 1007 a and 1007 b are each controlled by an n-channel transistor 1008 and state machine 107 of FIG. 9. Transistors 1007 a and 1007 b of current mirror block 1005 are controlled by state machine 107 through a p-channel transistor 1009.

[0049] To emulate the bias currents predicted for worse case stress conditions, state machine 107 steps any or all of the bias current BIAS1, BIAS2, BIAS3, and BIASLPF, from their base values generated by the current mirrors of transistors 106 a and 106 b of current mirror blocks 1004 a-1004 c and 1005, by activating the corresponding transistors 1008 or 1009. For example, to step the bias current BIASI1, state machine 107 of FIG. 9 activates transistor 1008 of current mirror block 1004 a, to step the bias current BIASLPF, state machine 107 activates transistor 1009 of current mirror block 1005, and so on.

[0050] The inverse logic of n-channel and p-channel transistors 1008 and 1009 allows the bias current to the low pass filter of attenuator-LPF block 1005 block to be decreased relative to the bias current to integrators I1-I3, and vice versa. The bias currents BIASI1, BIASI2, BIASI3, and BIASLPF are stepped simultaneously, sequentially, or in any combination required to induce the worst case bias current conditions in amplifier 900 as predicted for a given stress on amplifier 900. Consequently, any marginal circuits within amplifier 900 are forced to fail under nominal test conditions which eliminates the need to generally test operational amplifier 900 under stress conditions.

[0051] The results of emulating stresses on the various blocks of operational amplifier 100 by varying the internal bias currents are preferrably detected using the voltage offset measurements described above, although in alternate embodiments other tests or measurements may be performed. Hence, when a set of bias currents are stepped by bias generator 901 to produce a predicted worst case set of operating conditions, the test blocks TEST1, TEST2, and TEST3 detect any out of specification offset voltages indicative of a marginal or failing circuit block.

[0052] As shown in FIGS. 11 and 12A-12F, state machine 107 of operational amplifier 900 (see FIG. 9) performs a series of tests at selected internal nodes, similar to those previously described with respect to operational amplifier 900, however, in conjunction with FIGS. 3 and 4A-4F. Amplifier 900, however, is further operable in the extended test mode shown in FIG. 11, which includes both normal test period, during which the tests described above are conducted with the nominal (base) bias currents, and a stress mode in which the bias currents are stepped in order to test under emulated worst case stress conditions. In this example, an additional millisecond is added to the test sequence time and state machine 107 generates the timing for an additional testmode period shown as Testmode3 in FIG. 12G. Alternatively, operational amplifier may be tested only in the stress mode or only in the normal test mode, depending on the test protocol.

[0053] Alternatively, one or more of the bias currents BIASI1, BIASI2, BIASI3, and BIASLPF is fixed during testing under nominal test conditions. In this case, potential failures under actual stress conditions are identified by varying (tightening) the test constraints imposed on the tested operating parameter. For example, predictions are made to determine a range of offset voltages measured at the output of a given amplifier stage 101 a-101 c which corresponds to the proper operation of that amplifier stage 101 a-101 c under stress conditions. Measurements of an offset voltage by the associated test block 105 a-105 c under nominal test conditions which exceed the predicted range are then deemed as indicating a failure of the associated amplifier stage 101 a-101 c.

[0054] Although the invention has been described with reference to a specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

[0055] It is therefore, contemplated that the claims will cover any such modifications or embodiments that fall within the true scope of the invention. 

What is claimed is:
 1. A method of testing an internal block of an integrated circuit comprising: initiating a test mode; in the test mode, testing an internal block of an integrated circuit under a first operating condition by setting a selected operating parameter to a value emulating operation of the internal block under a second operating condition to detect potential failure of the internal block under the second operating condition; and selectively outputting a test signal from a selected pin of the integrated circuit in the test mode indicative of the operation of the internal block, wherein the selected pin is utilized for exchanging another signal when the integrated circuit is in another operating mode.
 2. The method of claim 1, wherein initiating the test mode comprises internally initiating the test mode automatically in response to a selected state of the integrated circuit.
 3. The method of claim 2, wherein the test mode is initiated automatically in response to a power-up state of the integrated circuit.
 4. The method of claim 1, further comprising determining the value by determining a value corresponding to expected operation of the internal block under the another operating condition.
 5. The method of testing of claim 1, wherein the selected operating parameter comprises a bias current.
 6. The method of testing of claim 1, wherein the stress comprises a deviation in operating temperature from nominal.
 7. The method of testing of claim 1, wherein the stress comprises a worst case operating temperature.
 8. The method of testing of claim 1, further comprising modulating a current to the integrated circuit in response to detecting a potential failure of the internal block under the stress.
 9. The method of testing of claim 1, testing the integrated circuit to detect potential failures of the internal block under stress comprises observing an offset voltage at a selected node coupled to the internal block.
 10. The method of testing of claim 1, wherein the internal block comprises one of a plurality of internal blocks of the integrated circuit and a selected parameter of each of the plurality of blocks is varied to a predicted value to emulate operating conditions under stress.
 11. An integrated circuit comprising: an internal circuit operating in response to a bias current; a bias current generator for selectively varying the bias current by an amount predicted to emulate operation of the internal circuit under a selected stress condition in a test mode; and test circuitry for testing the operation of the internal circuit with the varied bias current, the test circuitry including circuitry for selectively outputting a test signal from a selected pin of the integrated circuit in the test mode indicative of the operation of internal circuit, wherein the selected pin is utilized for exchanging another signal when the integrated circuit is in another operating mode.
 12. The integrated circuit of claim 11, further comprising circuitry for internally initiating the test mode automatically in response to a selected state of the integrated circuit.
 13. The integrated circuit of claim 12, wherein the selected state of the integrated circuit is power-up.
 14. The integrated circuit of claim 11, wherein the test circuitry comprises a state machine operable to control variation of the bias current by the bias current generator during stress mode testing.
 15. The integrated circuit of claim 14, wherein the state machine is further operable to maintain the bias current at a nominal value during normal mode testing.
 16. The integrated circuit of claim 11, wherein the bias current generator comprises a first current mirror for generating a base bias current to the internal block and a second current mirror to step the base bias current by the predicted amount.
 17. The integrated circuit of claim 11, wherein the test circuitry observes an offset voltage at a test node associated with the internal circuitry.
 18. The integrated circuit of claim 11, wherein the internal circuit comprises a selected stage in a multiple-stage device and the bias current generator is operable to selectively independently vary bias currents to selected stages in the multiple stage device.
 19. An amplifier comprising: a plurality of stages each operating in response to a corresponding bias current; a bias current generator operable during a first mode to provide the corresponding bias currents to the plurality of stages and to selectively vary the corresponding bias currents during a second mode to emulate operating stress; and test circuitry for observing selected nodes between the plurality of stages in the second mode, the test circuitry automatically initiating the second mode in response to a state of the amplifier.
 20. The amplifier of claim 19, wherein the test circuitry selectively varies a supply current to the amplifier in the second mode to represent a test result indicative of the operation of the plurality of stages.
 21. The amplifier of claim 19, wherein the bias current generator selectively varies the corresponding bias currents by selected amounts predicted to emulate operations of the plurality of stages under the operating stress.
 22. The amplifier of claim 19, wherein the first mode comprises a normal operating mode and the second mode comprises a test mode.
 23. The amplifier of claim 19, wherein the first mode comprises a normal test mode and the second mode comprises a stress test mode.
 24. The amplifier of claim 19, wherein the test circuitry is operable to observe an offset voltage at the selected nodes.
 25. A method of testing an internal block of an integrated circuit comprising: determining a test limit for an operating parameter of an internal block operating under an operating condition; automatically internally initiating a test mode for testing the internal block; and testing the integrated circuit in the test mode under another operating condition to determine if the selected operating parameter exceeds the test limit.
 26. The method of claim 25 wherein determining the test limit comprises determining an expected limit for the operating parameter for the internal block operating under a stress operating condition.
 27. The method of testing of claim 25, wherein the selected operating parameter is an offset voltage of the internal block.
 28. The method of testing of claim 25, wherein the another operating parameter is a bias current to the internal block.
 29. The method of testing of claim 25, wherein predicting a value limit comprises predicting a range of values of the selected operating parameter corresponding to proper operation of the internal block under the stress.
 30. The method of testing of claim 25, wherein predicting a value limit under the stress comprises predicting a value limit on the selected operating parameter under selected temperature conditions. 